Here's why:

* Stratus clouds form close to the ground, usually below 2,000 feet. They appear as a flat, gray sheet that covers the entire sky.

* Other low-level clouds include cumulus and stratocumulus, but they generally form at slightly higher altitudes than stratus clouds.

However, it's worth noting that sometimes fog can be considered a type of cloud that forms at ground level. So, if you're looking for the absolute lowest cloud formation, fog would technically win!